The Impact of Synchrotron Radiation in the LHC · 2013. 1. 14. · Critical energy of SR eV 7 105...

O. Gröbner CERN-LHC/VAC VLHC Workshop 18 - 20 Sept. 2000

The Impact of SynchrotronRadiation in the LHC

O. GröbnerCERN-LHC


Credits

• LHC Vacuum Group and SL/AP Group– V, Baglin, O. Brüning, I.R. Collins, P. Cruikshank, C. Gruenhagel,

B. Henrist, N. Hilleret,. B. Jenninger, M. Jimenez, N. Kos, J.-M. Laurent,F. Le Pimpec, P. Lepeule,M. Pivi, O. B. Malychev, G. Moulard, A. Rossi,F. Ruggiero, G. Schneider, P. Strubin, R. Veness, F. Zimmermann

• BINP– V.V. Anashin, R. V. Dostovalov, N.V. Fedorov, N.A. Gorbunova,

I.A. Koop, A.A. Krasnov, O. B. Malychev, V.P.Naz’mov, V.N. Osipov,V.F. Pindyurin, E.E. Pyata

• LBL (SSC)– W. Turner


Synchrotron Radiation Effects• S.R. Power deposition on cryogenic system• Photon stimulated outgassing -> dynamic pressure increase• Photoelectron production -> electron cloud effect

– Power dissipation by bunched beam– Beam Induced Multipacting (BIM)– Electron stimulated outgassing

• Beneficial effects of S.R. :– Cleaning of the vacuum system (beam cleaning effect)– Reduction of secondary electron coefficient by photons/electrons

• (beam scrubbing)– For the VLHC : Radiation damping -> Increase of Luminosity


Comparison of S.R. Characteristics

LEP200 LHC SSC HERA VLHC

Beam particle e+ e- p p p p

Circumference km 26.7 26.7 82.9 6.45 95

Beam energy TeV 0.1 7 20 0.82 50

Beam curre nt A 0.006 0.54 0.072 0.05 0.125

Critical energy of SR eV 7 105 44 284 0.34 3000

SR power (total) kW 1.7 104 7.5 8.8 3 10-4 800

Linear power density W/m 882 0.22 0.14 8 10-5 4

Desorbing photons s-1 m-1 2.4 1016 1 1017 6.6 1015 none 3 1016


Heat load contributions in the LHC


Beam induced heat loads

1) Resistive lossesP ∝ ρw I

2 ≤ 0.05(W /m)

2) Synchrotron radiation, (s.r.)

P(W /m) = 1.24⋅103 E4(TeV) I( A)

ρ2(m)3) Photoelectrons

s.r. creates an electron cloud ->Without secondary electrons P ~ Nb3

with secondary electrons -> limited by space charge

4) Nuclear scattering

P(W /m) = 0.93I (A) E (TeV)

τ(h)Beam screen removes #1, #2 and #3 and #4 sets a limit forthe beam lifetime


Beam screen inside a Cryo-magnet cold bore


LHC-beam screen withpumping holes and coppercoating (0.05 mm).

Cooling capillaries areprovided to remove beam-induced power:-> Synchrotron radiation-> Beam image currents-> Photo-electrons


LHC Twin-aperture cryomagnet

Beam vacuumBeam screen


Beam line at DCI and EPA forSR-induced molecular desorption studies

RGA

P2

P1P3

Ti/SP

B.A. Gauge

Gas inlet

Calibratedconductance

B.A. Gauge

Photon beam

Test chamber

Photoelectroncollector

Collimator

Ti-sublimation+ Ion pump

Shutter

Valve

Turbo +Rotary-Pump

DCIRing


Cryogenic experiment at BINP on VEPP2-M


Cold bore experiment (COLDEX) at CERN

LHe < 3 K

Bea m sc re en

Cold bore ve ss el

The rma l scre e n

V

LHe @ 4 .2 K

GHe @ 4 .2 - 6 0 K

GHe @ 4 .2 K

Gat e va lve

Collim at or

SR ligh t

He at er sVa lve s

Pumping unit s

Va cuum ga uge s

Le ve l s ensorsTe mpe ra ture se ns ors

Supports w ith t ransla tion ta b le (la te ra l m ove me nt to align be am sc re en t o s ync hr o tron ra dia t ion be am )

Expe rim ent al cr yost a t

Phas e Se par at or

LHe Dewar

Ga s sh ie lde d LHe - tra nsfe r line ( 3 4 m )

va cuum insu lat e d tra nsfe r line (2 m)

Inside EPAOuts ide EPA

Helium re co ve ry ne t work

Ma ss spe ct rome t er

Conduc ta nce

F F

Buff er

Experim e nt a l vac uum (be a m va cuum)

V

V

Insu la tion va cuum

Be am shutt e r

V

V

M

M

M

V

Mass flow met e rs

VVV

V

P

PP

P

Pr es sure s ensors


COLDEX in EPA beam line


10-6

10-5

10-4

10-3

10-2

1018 1019 1020 1021 1022Dose (photons/m)

H2

CO

CO2CH4

0.0001

0.001

0.01

0.1

1

1015 1016 1017 1018 1019 1020

CO2 ~68K CO2 5.5-20KCO 5.5-15KCH4 5.5-20K

H2 3KH2 [4]H2 [3]

Average coverage (molecules/cm2)

Primary photo-desorption coefficient and re-cycling coefficientof physisorbed gas molecules


Molecular desorption yield vrs desorbed gas load

10-8

10-7

10-6

10-5

10-4

10-3

10-2

10-5 10-4 10-3 10-2 10-1 100

etaH2etaCH4etaCOetaCO2E

TA

(mol

ecul

es /

phot

on)

Desorbed molecules (Torr l / m)

OFHC copper, baked. With 3.75 keV critical energy synchrotron radiation


Variation of the desorption yield with critical photon energyof synchrotron radiation spectrum.

10-5

10-4

10-3

10-2

10-1

101 102 103 104 105 106

EPAINPDCILEP

Des

orpt

ion

yiel

d (a

rbitr

ary

units

)

Critical photon energy (eV)

Data compiled for different materials (Al, OFHC copper and stainless steel) and for different machines


Molecular desorption yield

oo

o

oo

1x10-5

1x10-4

1x10-3

1x10-2

0 2 4 6 8 10 12 14Beam energy (TeV)

o H2CO

CO2

Photon induced moleculardesorption yield as afunction of the LHC beamenergy.

Data obtained withunbaked, electroplatedcopper on stainless steel.Measured at roomtemperature in the photonbeam line at EPA in CERN

EVC 95, Uppsala

LHC


Dynamic Vacuum model for the LHC

Vdndt

= q − a n + b θ Fdθdt

= c n − b θ

a =14

v F (s + f )

b = F νo e−

E

kT + κ Ý Γ

c =14

v s F θ =cb

n =14 v s F

Fνo e− E kT +κ Ý Γ

n

q photon induced desorption ratea pumping by the wall and by the pumping holesb physisorbed molecules desorb thermally and by photonsc adsorption rate proportional to sticking probability, s

and


0.E+00

1.E-10

2.E-10

3.E-10

4.E-10

5.E-10

0.E+00 1.E+21 2.E+21 3.E+21 4.E+21 5.E+21 6.E+21 7.E+21

Dose (Photons/m)

To 52 K

H2

CO

CO20.E+00

1.E-08

2.E-08

3.E-08

4.E-08

-2 0 2 4 6 8 10 12

Time (h)

~ 3.2 10 15 H2/cm2

~ 1.6 10 15 H 2/cm2

0.E+00

2.E-10

4.E-10

6.E-10

8.E-10

1.E-09

0.0E+00 5.0E+20 1.0E+21 1.5E+21 2.0E+21 2.5E+21

Dose (Photons/m)

Without holes

With holes

COLDEX-Results at EPA between 3K to 20 K

With pumpingholes

withoutpumping holes

Pre-condensed H2


Hydrogen vapour pressure

10-12

10-11

10-10

10-9

10-8

10-7

10-6

0 1 2 3 4 5 6 7

P 4.17kP 3.6KP 3.37KP 3.07KP 2.79KP 2.3K

Pre

ssur

e (T

orr)

Coverage (H2 molecules cm -2 10 15 )

4.71 K

3.6 K

3.37 K

3.07 K

2.79 K

2.3 K

C. Benvenuti, R. Calder


Hydrogen vapour pressure


Effect of temperature variations on the pressureCOLDEX #50 30-31/8/2000, 194 eV, 217.7 mA,

Cu BS no holes. BS = 7.0 K + oscillations, CB = 141.0 K

1.E-10

1.E-09

1.E-08

1.E-07

1.E-06

0 0.5 1 1.5 2 2.5 3

Time (h)

0

10

20

V. Baglin, CERN LHC-VAC


Hydrogen vapour pressure due to room temperature radiation

C. Benvenuti, R. Calder, G. Passardi, J.Vac. Sci. Technol.,Vol 13, No. 6, Nov./Dec. 1976, 1172

LHC beam screen andat nominal conditions :synchrotron radiation ->~0.15 mW/cm2

For the LHC: the densitylimit of 1015 H2 m-3 wouldcorrespond at 2.3 K to alimit pressure of

Plimit = 2.4 10-10 Torr

LHC cold bore mustbe screened fromsynchrotron radiationto cryo-sorb a largecoverage of hydrogenwith a low vapourpressure


Electron cloud generated heat loadand outgassing in the LHC beam screen

δ(E,φ)

Y(E,φ)

γ

Reflection

Secondary electrons

Key parametersSynchrotron radiation intensity,Y(E,φ) photoelectric yieldδ(E,φ) secondary electron yield, Emax, δmaxEnergy distribution of secondary electronsPhoton reflectivity (magnetic field)Beam screen shape and diameterBunch intensity and spacingExternal fields (magnetic, electric, spacecharge)

Qcloud = kηe Plin

< Ecloud >Outgassing:k converts molecules to pressure average energyηe molecular desorption yieldPlin (W/m) linear power deposition


Energy deposition by the electron cloud

• Beam induced power must beintercepted by the activelycooled beam screen

• Electron cloud effect– Beam blow-up– Multipacting -> BIM

Energy gain of electrons by the passageof an LHC bunch as a function of theradial position in the beam pipe.


Effect of a dipole magnetic field

B

φF

Primary SR photons strike in themedian plane. Here photoelectrons aresuppressed by the magnetic field.

Reflected/scattered photons reach topand bottom of the beam pipeLow photon reflectivity is desirable toreduce the photoelectrons which canmove freely along the field lines.

Electron

Cyclotron oscillations/bunch ~120Cyclotron radius ~ 6 µm for 200 eV

F is the electric force excerted by theproton bunch. on the electron.

ElectronBeam


Electron cloud in a dipole magnetic field

LHC beam screen(quadrant)

Average energy of theelectrons as a function of thehorizontal position from thebeam in an LHC dipole.

Pumping slots


Beam scrubbing in a dipole field

Secondary electron yieldBefore and after scrubbing

Averaged yield vrs. Radial positionfor different bunch intensities


Beam screen pumping slot pattern

Power deposited on cold bore:Inner row at 5 mmOuter row at 9 mm from center line

60 slots/m with an average length of 8mmRepresenting a transparency of ~ 0.48

At nominal beam parameters :Inner row ~ 5%Outer row ~ 2% of the energy deposited uniformly by the electron cloudmay be reach the cold bore.

Hence, for 0.5 W/m -> 35 mW/m


Beam Induced Multipacting (BIM)

– Pressure Increase due to Multipacting• Electron stimulated gas desorption has been

observed in several machines : first in ISR in1977 recently in KEK-B, PEP-2, SPS withLHC-type beams.

• Gas load, Qcloud is directly related to the powerdeposited by the electrons, Plin, to themolecular desorption yield,

• ηe and to the average energy of the electronsin the cloud, Ecloud .

Qcloud=kηePlin


Electron stimulated molecular desorption

K. Kennedy, LBL,unpublished note, 1986


Beam Induced Multipactingwith an LHC-type beam in the SPS

0

1

2

3

4

5

6

7

8

9

0.0E+00 1.0E+12 2.0E+12 3.0E+12 4.0E+12 5.0E+12 6.0E+12 7.0E+12Number of protons

mba

r in

crea

se

VG50660 Dipole magnetic fieldVG51880 No magnetic field


Reduction of BIM in the SPS with running time

y = 12e-0.9 x

1

10

100

0:00 12:00 24:00 36:00 48:00 60:00Running hours

VG50660

VG52260

y = 10e-0.9 x

Gauges placed between dipoles


Secondary electron yield

• Secondary electron yield oftechnical copper and aluminiumsurfaces

• Due to dosing with electronsand photons, the yield decreasesto between 1 to 1.4

• Secondary electron yield forCu, initial values and after aphoton dose of 1.5 1020photons/m at 194 eV criticalenergy.

• Emax and δmax are input forsimulation

0.00

0.50

1.00

1.50

2.00

2.50

3.00

3.50

4.00

0 500 1000 1500 2000

Aluminium

Copper OFHC

Seco

ndar

y el

ectr

on y

ield

Energy (eV)


Reduction of the secondary electron yield by photonscrubbing

MAX SEY vs photon dose at EPACritical energy 194 eV

0.00

0.50

1.00

1.50

2.00

2.50

1.E+18 1.E+19 1.E+20 1.E+21 1.E+22 1.E+23 1.E+24

Photon dose (ph/m)

100 V

-45 V

350 V

Sample at - 45 V

Sample at + 100 V

Sample at + 350 V

~ 1 year of nominal LHC operation


Compilation of photoelectric yields and ofphoton reflectivities

308 MeV 500 MeVDate Tube status R (%) Y*(e/ph) R (%) Y*(e/Ph)7/2/97 Cu roll bonded Unbaked 80.9 0.114 77 0.318

8/28/97 Cu electroplated Unbaked 5 0.084 6.9 0.0789/17/97 Cu roll bonded air baked Unbaked 21.7 0.096 18.2 0.18011/13/97 Cu "macro" ribbed Unbaked 1.8 0.053 - -11/14/97 150, 9 hours 1.3 0.053 1.2 0.05211/17/97 150, 24 hours 1.3 0.040 1.2 0.04011/18/97 Ti Zr Unbaked 20.3 0.055 17.1 0.07811/19/97 120, 12 hours 19.5 0.048 16.7 0.07211/20/97 250, 9 hours 19.9 0.026 17.4 0.04011/25/97 350, 10 hours 20.6 0.015 16.9 0.028

11/28/97350, 10 h aftersaturation with5E15 CO/cm2

20.7 0.016 - -

12/1/99 Cu colaminated sawtooth Unbaked - 0.029 6 0.038

I beam current, R forward reflectivity of photonsFcoll = 0.46 at 308 MeV and 0.65 at 500 MeV due to photon collimationLhit = 3.414 m, Lcollected = 0.5 m calculated

Γ.

[photons.s -1 ] = 1.0029 10 15 IBeam [mA] E Beam [GeV]Y* [e.photon -1] = I

q

Γ.

FCollimator (1 - R)

LHitLCollected

V. Baglin, LHC-VAC


Reduction of photoelectric yield by photon scrubbing

1.5 10 -2

2.0 10 -2

2.5 10 -2

3.0 10 -2

3.5 10 -2

4.0 10 -2

1019 1020 1021 1022 1023 1024

Phot

olec

tric

yie

ld (e

lecr

ons/

phot

on)

Photon dose (photons/m)

Exposures to N2

Copper with 'saw-tooth' structure194 eV critical energy

V. Baglin, LHC-VAC


“Saw-tooth” surface inprinted into Cu layer

Grazing angle of incidence of photons in the LHC arc: 2 to 3 mrad


Experimental setup in INP

Set-up to measure the distribution of reflected Photons as well as photo electron production from a test beam screenIn the presence of a magnetic dipole field


Reduction of the secondary electron yield bybeam scrubbing in LHC

Required scrubbing time t =2π rp D

e Rγ ΓD scrubbing dose [C/m2]rp beam pipe radiuse unit chargeR photon reflectivity, (in magnetic field)γ photoelectric yieldΓ linear photon flux

Scrubbing dose for δmax


Nominal heat loads on LHC beam screen

Without Sawtooth

With Sawtooth

Initial Final Initial Final

Electron CloudImage CurrentsSynchrotron RadiationHeat Inleaks

0

1

2

3

4

5

6


Summary for LHC

• Design of the LHC vacuum system is strongly influencedby s.r. effects.

• Primary and secondary desorption effects• Total quantity of desorbed gas -> saturated vapour pressure• Physisorbed gas molecules must be screened from effects

of the beam (s.r. but also (photo-)electrons and ions)• Temperature fluctuations of cryosorbing surfaces (beam

screen) must be avoided or minimised• Secondary desorption -> re-distribution of cryosorbed gas

and important transient effects.• S.R. produces large number of photoelectrons -> e-cloud

effects.

The Impact of Synchrotron Radiation in the LHC · 2013. 1. 14. · Critical energy of SR eV 7 105...

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